Mechanical parts such as automobile parts are usually manufactured by hot-forging a steel bar to prepare mechanical parts having a prescribed shape, and then applying a refining heat treatment comprising hardening and tempering, to the thus prepared mechanical parts.
The above-mentioned refining heat treatment, which is applied for the purpose of imparting desired toughness and strength to the mechanical parts, requires largescale facilities and a huge thermal energy. If, therefore, the above-mentioned refining heat treatment can be omitted from the manufacturing process of the mechanical parts, it would permit simplification of the facilities and saving of thermal energy.
As a steel bar not requiring the above-mentioned refining heat treatment after preparation of a steel article, i.e., as a non-refining steel bar, the following ones have conventionally been proposed:
(1) A non-refining steel bar, disclosed in Japanese Patent Provisional Publication No. 59-100,256 dated June 9, 1984, which comprises:
carbon: from 0.20 to 0.40 wt. %, PA1 silicon: from 0.01 to 1.50 wt. %, PA1 manganese: from 0.8 to 2.0 wt. %, PA1 vanadium: from 0.01 to 0.20 wt. %, PA1 nitrogen: from 0.002 to 0.025 wt. %, PA1 aluminum: from 0.001 to 0.05 wt. %, PA1 sulfur: up to 0.05 wt. %, PA1 titanium/nitrogen: from 0.2 to 2.5, PA1 carbon: from 0.05 to 0.15 wt. %, PA1 silicon: from 0.10 to 1.00 wt. %, PA1 manganese: from 0.60 to 3.00 wt. %, PA1 aluminum: from 0.01 to 0.05 wt. %, PA1 carbon: from 0.05 to 0.18 wt. %, PA1 silicon: from 0.10 to 1.00 wt. %, PA1 manganese: from 0.60 to 3.00 wt. %, PA1 titanium: from 0.010 to 0.030 wt. %, PA1 boron: from 0.0005 to 0.0030 wt. %, PA1 aluminum: from 0.01 to 0.05 wt. %, PA1 nitrogen: up to 0.0060 wt. %, PA1 carbon: from 0.06 to 0.15 wt. %, PA1 silicon: from 0.10 to 1.00 wt. %, PA1 manganese: from 0.50 to 2.00 wt. %, PA1 titanium: from 0.010 to 0.030 wt. %, PA1 boron: from 0.0005 to 0.0030 wt. %, PA1 aluminum: from 0.01 to 0.05 wt. %, PA1 carbon: from 0.020 to 0.049 wt. %, PA1 silicon: from 0.10 to 1.00 wt. %, PA1 manganese: from 1.00 to 3.50 wt. %, PA1 chromium: from 0.50 to 3.50 wt. %, PA1 vanadium: from 0.02 to 0.20 wt. %, PA1 aluminum: from 0.01 to 0.05 wt. %, PA1 heating said material to the austenization temperature region; PA1 hot-working said material in the austenization temperature region to prepare a steel article; and PA1 cooling said steel article thus prepared from the austenization temperature region to a temperature of or lower than 300.degree. C. at a cooling rate of from 2 to 100.degree. C./second, thereby imparting a high toughness and a high strength to said steel article. PA1 boron: from 0.0003 to 0.0030 wt. %, PA1 titanium: from 0.005 to 0.030 wt. %. PA1 nickel: from 0.05 to 1.00 wt. %, PA1 copper: from 0.05 to 1.00 wt. %, PA1 molybdenum: from 0.05 to 0.50 wt. %, PA1 niobium: from 0.005 to 0.050 wt. %.
and
the balance being iron and incidental impurities (hereinafter referred to as the "Prior Art 1").
(2) A non-refining steel bar, disclosed in Japanese Patent Provisional Publication No. 60-103,161 dated June 7, 1985, which comprises:
where, the total amount of manganese and chromium being from 2.20 to 5.90 wt. %,
and
the balance being iron and incidental impurities (hereinafter referred to as the "Prior Art 2").
(3) A non-refining steel bar, disclosed in Japanese Patent Provisional Publication No. 61-19,761 dated Jan. 28, 1986, which comprises:
where the total amount of manganese and chromium being from 1.60 to 4.20 wt. %,
and
the balance being iron and incidental impurities (hereinafter referred to as the "Prior Art 3").
(4) A non-refining steel bar, disclosed in Japanese Patent Provisional Publication No. 61-139,646 dated June 26, 1986, which comprises:
where, the total amount of manganese and chromium being from 2.00 to 4.00 wt. %,
and
the balance being iron and incidental impurities (hereinafter referred to as the "Prior Art 4").
The above-mentioned Prior Arts 1 to 4 have the following problems. More particularly, in the Prior Art 1, which permits achievement of a higher strength by adding vanadium and of a higher toughness by adding titanium, the high carbon content of from 0.20 to 0.40 wt. % imposes a limit in increasing toughness. In the Prior Arts 2, 3 and 4, which permit achievement of a higher strength as compared with the Prior Art 1, toughness is equal or inferior to that in the Prior Art 1. Particularly in the Prior Art 4, the high carbon content of from 0.06 to 0.15 wt. % poses difficulties in toughness.
Under such circumstances, there is a strong demand for development of a method for manufacturing a steel article having a higher toughness and a higher strength than in the above-mentioned Prior Arts 1 to 4, i.e., having a toughness including a Charpy impact value at 25.degree. C. (uE25.degree. C.) of at least 15.0 kgf.m/cm.sup.2 and a Charpy impact value at -40.degree. C. (uE-40.degree. C.) of at least 10 kgf.m/cm.sup.2 and having a strength including a yield strength (YS) of at least 60 kgf/mm.sup.2 and a tensile strength (TS) of at least 80 kgf/mm.sup.2, but such a method has not as yet been proposed.